“Visitors to White Island are now at the highest level of risk since the start of the 2012 eruptions," Jolly said.

"Hazards to visitors can include the health effects of volcanic gas exposure, including respiratory issues, skin and eye sensitivity to acid gases.

"Explosive eruptions can occur at any time with little or no warning. We advise a high level of caution should be taken, if visiting the island.”

Scientists are hoping a monthly flight over White Island to sample volcanic gases will give them a clearer picture of what is happening inside the increasingly restive volcano.

Vulcanologist Brad Scott visited the island earlier this week and found hydrothermal activity in the small crater lake was some of the most vigorous he had seen since the late 1990s or early this century.

"This type of activity usually leads to stronger volcanic activity and is a significant concern," he said.

GNS Science said the state of unrest at White Island was increasing and future eruptions were possible with little or no warning.

Scott hoped the next monthly gas monitoring flight, due tomorrow provided weather conditions were right, would add more pieces of the puzzle to help understand what the volcano was up to.

"The flight measured the amount of carbon dioxide, hydrogen sulphide, and sulphur dioxide above the volcano. Those levels were compared to previous levels," he said.

If the gas levels were increasing it could indicate an increase in the amount of molten material coming into the volcano, or that the molten material there was moving shallower, Scott said.

But "it's a very complex story", and scientists took a range of other factors into account, including surface activity, seismic activity, and ground deformation, to build up a picture of how a volcano was behaving.

It was thought the activity on White Island was driven purely by steam, but if the gas monitoring results turned up something unexpected it could indicate that was not the case.

Scott said that in a geothermal system the temperature was just a little over 100 degrees Celsius, while with a magmatic system it could get up to 800C or 900C.

"It adds a lot more energy... and increases the likelihood significantly of eruption."